An internal combustion spark ignition engine may be operated at an air-fuel ratio that is lean of a stoichiometric air fuel ratio shortly after an engine is started and engine temperature is near ambient temperature. By operating the engine lean, hydrocarbon emissions from the engine may be reduced at a time when efficiency of a catalyst coupled to the engine is low so that vehicle emissions may be reduced. Further, since engine temperature is near ambient temperature, engine NOx emissions may be low while the engine operates with a lean air-fuel mixture. However, if the engine's air-fuel ratio is leaner than desired, the engine may misfire and engine emissions may increase. Alternatively, if the engine's air-fuel ratio is lean, but richer than is desired, engine hydrocarbon emissions may increase.
One way to ensure that the engine is operating within a desired air-fuel range is to position an oxygen sensor in an exhaust system coupled to the engine and adjust the engine air-fuel ratio based on oxygen sensor output. However, accurate oxygen sensor output may not be available until the oxygen sensor reaches a threshold temperature. Engine air-fuel ratio may be open loop controlled (e.g., controlled without feedback) during cold engine starting, but the actual engine air-fuel ratio may deviate from the desired engine air-fuel ratio due to fuel injector transfer function errors and other conditions. Variation in fuel system components may be compensated via fuel multipliers that are learned and adapted at warm engine operating conditions. Nevertheless, an engine's air-fuel ratio may deviate from a desired engine air-fuel ratio when an engine at or near ambient temperature is started due to fuel puddling and temperature dependent fuel component variation. Therefore, it may be desirable to provide a way to control engine air-fuel ratio during conditions when oxygen sensor feedback is not available.
The inventors herein have recognized the above-mentioned disadvantages and have developed an engine operating method, comprising: receiving sensor data to a controller; and adjusting an engine air-fuel ratio in response to a difference between a desired standard deviation of engine crankshaft acceleration and a determined standard deviation of engine crankshaft acceleration via the controller, the desired standard deviation of engine crankshaft acceleration based on a desired engine air-fuel ratio, the determined standard deviation of engine crankshaft acceleration based on the sensor data.
By adjusting engine air-fuel ratio in response to a difference between a desired standard deviation of engine crankshaft acceleration and a determined standard deviation of engine crankshaft acceleration, it may be possible to provide the technical result of improving engine emissions during times when oxygen sensor feedback is not available. In one example, the desired standard deviation of engine crankshaft acceleration may be based on a desired engine air-fuel ratio so that when the engine is operated at the desired standard deviation of engine crankshaft acceleration the engine operates at the desired air-fuel ratio. If the determined standard deviation of engine acceleration is less than or greater than desired, it may be judged that the engine is not operating at the desired air-fuel ratio. As a result, the engine air-fuel ratio may be adjusted. In this way, the engine may be operated at the desired air-fuel ratio without oxygen sensor feedback.
The present description may provide several advantages. In particular, the approach may reduce engine emissions during engine starting. Further, the approach provides for air-fuel ratio feedback when oxygen sensors may not be available to improve engine operation. Further still, the approach may make it possible to operate an engine with a smaller catalyst while meeting emissions requirements.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.